In order to measure a single photon level light emission phenomenon in the range of subnanoseconds, a measuring device with a photomultiplier according to a single photon counting method has been employed in the art.
FIG. 3 is a schematic diagram showing a conventional device for measuring a light emission phenomenon according to a single photon level counting method. In FIG. 3, specimen 1 is a vegetable photosynthesis coloring matter. The specimen 1 is repeatedly excited by the exciting light 2 which is a laser pulse train having a pulse width of the order to 10 p sec. A part of the exciting light 2 is received by a photo-diode 3 so that it is converted into an electrical signal.
The electrical signal is amplified by an amplifier 8 and subjected to waveform shaping by a constant fraction discriminator 12, to provide a reference signal pulse. The jitter of the reference signal pulse is of the order of 10 p sec.
On the other hand, a single photon level fluorescent light emitted by the specimen 1 that is excited by the exciting light 2 is applied through an optical system 6 to a photomultiplier 30. The fluorescent light is detected by the photomultiplier 30 and amplified by an amplifier 7. The output of the amplifier 7 is applied to a constant fraction discriminator 11, to form a pulse whose jitter is of the order of 200 p sec. The pulse is applied to a time-to-pulse height converter 14. On the other hand, the aforementioned reference signal pulse is sufficiently delayed by a delay circuit 12a so that the time difference between the pulse outputted by the constant fraction discriminator 11 and the reference signal pulse is smaller than the full scale of the time-to-pulse height converter 14. The pulse thus delayed is also applied to the time-to-pulse height converter 14, so that a voltage corresponding to the time difference between those pulses is outputted. The voltage thus outputted is applied to a pulse height analyzer 31.
In the pulse height analyzer 31, the voltage corresponding to the time difference is quantized, and the output is accumulated a plurality of times for each quantizing position. A graph is formed by plotting the time intervals on the horizontal axis and the degrees obtained through a number of exciting operations on the vertical axis. In this case, the light emission time characteristic of the specimen 1 excited by the exciting light 2 can be measured with a time resolution of the order of 200 p sec.
It can be considered that all the parts of a specimen do not emit the same weak light beams having the same attenuation waveforms. Therefore, when the above-described measuring device is used to measure the weak light beams of all the parts of a wide specimen, it is necessary to perform the above-described measurement for each of the parts on the premise that the specimen is maintained unchanged in characteristic. This is not a valid assumption and leads to inaccurate results.
Another type of photon imaging device that has a micro-channel plate electron multiplier is described in U.K. patent application GB 2 126 043A published on Mar. 14, 1984, and assigned to the present assignee.